The rapid adoption of solar PV across the rooftops of Australia is causing anxiety on parts of the grid. As penetration of solar households reaches more than 35% in parts of South Australia and Queensland, for example, distribution networks are researching ways to manage systems so that power quality is maintained and PV system owners get value from their investments.

Presenting during the All-Energy 2020 session Integrating DER into the Grid, Enea Consulting managing partner Oliver Lacroix went over the main findings so far from an ARENA-funded research project with Victorian distribution networks CitiPower and Powercor to assess the hosting capacity of low-voltage networks.

After isolating pain points on the networks the researchers built a “power flow model”, or digital twin, of the affected areas to simulate increasing levels of PV over a one-year period. They found six areas out of 10 surveyed were in breach of 40% PV penetration for more than eight hours a day (see chart). It’s a problem that will only get worse as homeowners buy new PV systems or upgrade old ones. The solution is to increase network hosting capacity, but how do you do that?

The team identified three methods at the network end to explore: increase taps (manually adjust voltage levels on transformers) or develop an on-load tap changer or low-voltage regulator.

At customer end, the researchers wanted to look at obvious solutions such as the rollout of smart inverters to optimise exports and the adoption of batteries to store surplus solar.

The results were informative. For the network, Lacroix said the best and cheapest solution is additional taps to lower voltage. Smart inverters significantly reduce voltage rise, he said, and offer the highest net benefit – but the payoff is curtailment. Perhaps surprisingly, residential storage was found to have no significant impact because household batteries typically charge before noon, the most critical point of the day for exports.

Lacroix offered six recommendations for managing network capacity: allow flexible/dynamic export limits; promote smart inverters; consider mitigation measures to complement smart inverters; upgrade transformers during replacement activities; build more power flow models for other networks, and; explore potential for behind-the-meter batteries.

The sun in South Australia

The problem of surging solar exports pushing demand to extreme lows is most evident in South Australia, where rooftop PV – at about 1.5GW – is the largest generator. For one hour on October 11, for example, PV supplied 100% of demand in the state.

SA Power Networks future networks program manager Alex Ward doesn’t necessarily feel this is a bad thing. “We continue to see more and more records broken, which is encouraging to see,” he said.

With exports limited to 5kW and one in three customers owning PV systems, SAPN is seeing congestion and localised high voltage issues in parts of its network. The network can only host 1-2kW of exports per customer, Ward said.

There are two choices: maximise hosting capacity with better voltage regulation, and/or; increase utilisation of hosting capacity through higher levels of self-consumption and smarter loads, such as scheduling charging of EVs and hot water systems “to soak up some of that generation in the network”.

The network is involved in an ARENA-funded virtual power plant to facilitate integration of DER into the grid, where Tesla batteries and inverters by Fronius, SMA and SolarEdge in 1,000 homes communicate using SwitchDin smarts in a coordinated effort to calm voltage levels.

The 5MW VPP is demonstrating its capability, Ward said, showing a chart for one household where a Tesla battery is charged and discharged to help stabilise energy market voltage levels. The flexible export limit for households varies throughout the day between 5kW and 10kW, and the VPP operators’ hopes are to extend the VPP from 5MW export capability to 10MW.

For the trial, SAPN built a standard web API to identify battery locations and monitor voltage and flexible export limits. It is working on a flexible exports offering for systems under 200kW.

“Flexible exports is a key and flagship enabler of more DER on the network,” Ward said.